1. Field of the Invention
This invention relates to apparatus for severing sections from a web by providing transverse severing cuts in the web at locations related to printed marks on the web. More particularly, the invention relates to web severing apparatus that includes a servomotor operable in dependence on the deviation of the spacing of the printed marks from a predetermined web section length.
2. Description of the Prior Art
In cutting apparatus that passes the position of printed marks on a web to initiate a cut, the spacing of the printed marks will inevitably differ to some extent from the desired length of the web sections which would be cut off if a correcting rotation was not imparted to the web feeding apparatus. In order to ensure that the cuts will be properly positioned relative to the printed marks on the web, the feed movement imparted to the web by the web feed rollers must be adapted to the variations from printed mark to printed mark in such a manner that the next cut will be properly related to the associated printed mark. In connection with the drive of the feed rollers it will be necessary to compensate a constant average deviation which is due to the difference which will be obtained between the average spacing of the printed marks and the length of the cut-off web sections if correcting rotation is not imparted to the web feeding apparatus by a servomotor, and it will be necessary to compensate that average deviation as well as the errors which are due to the different spacings between successive printed marks.
Published German Patent Application No. 20 02 445 discloses a web cutting apparatus in which the constant average deviation is compensated by a constant adjustment of an infinitely variable transmission, which consists of a double-cone belt drive and is connected between the drive means for the cutter roller and a differential transmission. The errors which are due to the different spacings of the printed marks are compensated by a correcting rotation which is imparted by a servomotor to the differential transmission for short periods of time. For this reason, it is necessary to provide not only the servomotor, which is directly connected to the differential transmission, but also an infinitely variable transmission, which is adjusted by a constant amount by means of another servomotor. In that apparatus, the printed marks are detected by a photocell and the output signals of the photocell are compared with the output signals of a cycle detector which is connected to the drive shaft of the rotary cutter. Pulses which represent the phase displacement are continuously counted by a counter, which generates final control signals delivered to the servomotors. The servomotor associated with the infinitely variable transmission will receive a final control signal only when pulses have been counted in a predetermined number, which indicates that a permanent adjustment is required.
The known apparatus includes a servomotor for imparting correcting rotation directly to the differential transmission so as to effect a continuous adjustment, as well as an infinitely variable transmission and an associated servomotor, which ensures that the frequency of the pulse train which is generated by the printed marks is approximately as high as the frequency of the pulses generated by the cycle detector associated with the rotary cutter. However, the known apparatus is relatively expensive because the drive means required for the automatic register control includes a differential transmission, two servomotors and an infinitely variable transmission.
It is an object of the present invention to provide a web severing apparatus that includes simpler and less expensive drive means.